Formation of high quality titania, alumina and other metal oxide templated materials through coassembly

ABSTRACT

A co-assembly method for synthesizing inverse photonic structures is described. The method includes combining an onium compound with a sol-gel precursor to form metal oxide (MO) nanocrystals, where each MO nanocrystal has crystalline and amorphous content. The MO nanocrystals are combined with templating particles to form a suspension. A solvent is evaporated from the suspension to form an intermediate or compound product, which then undergoes calcination to produce an inverse structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of U.S. patentapplication Ser. No. 16/089,805, filed Sep. 28, 2018, now U.S. Pat. No.11,192,796, which is a National Stage Application of PCT InternationalApplication No. PCT/US17/25721 filed Apr. 3, 2017, which claims thebenefit of U.S. Patent Application No. 62/316,772, filed Apr. 1, 2016,all of which are incorporated in their entirety by reference.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with government support under N66001-11-1-4180awarded by the U.S. Department of Defense/DARPA, under DE-AR0000326awarded by the U.S. Department of Energy/ARPA-E, and underN00014-11-1-0641 awarded by the U.S. Department of Defense/ONR.

INCORPORATION BY REFERENCE

All patents, patent applications and publications cited herein arehereby incorporated by reference in their entirety in order to morefully describe the state of the art as known to those skilled therein asof the date of the invention described herein.

FIELD OF THE INVENTION

The present application relates to the formation of high-qualitytemplated materials. More particularly, the present application relatesto mixed amorphous-crystalline precursors for templating metal oxidestructures, for example inverse opals, using co-assembly.

BACKGROUND

Inverse replicas of opals, or “inverse opals,” comprise regulararrangements of pores that collectively exhibit light reflectiveproperties and methods of making the same.

SUMMARY

In accordance with certain embodiments, a co-assembly method forsynthesizing templated materials, such as inverse photonic structures,is described. The method includes combining an onium compound with asol-gel precursor to form metal oxide (MO) nanocrystals, where each MOnanocrystal has crystalline and amorphous content (i.e., a crystallinephase and an amorphous phase). The MO nanocrystals are combined withtemplating particles to form a suspension. The combining can comprisearranging the templating particles into a direct opal structure. Asolvent is evaporated from the suspension to form an intermediate orcompound product. The intermediate or compound product can then becalcined to produce a templated material, such as an inverse opal orother inverse structure. The templated material can be a photonicstructure.

In accordance with certain embodiments, the onium compound is aquarternary ammonium salt, for example an alkyl ammonium hydroxide suchas tetramethyl ammonium hydroxide (TMAH). A molar ratio of the oniumcompound to the sol-gel precursor can be at least about 0.05, forexample between about 0.3 and about 1.85. Combining the onium compoundwith the sol-gel precursor can be performed in a liquid, which cancomprise at least one of an aqueous solvent, an organic solvent, and amixed solvent.

In accordance with certain embodiments, the sol-gel precursor includestitanium isopropoxide (TIP), aluminum isopropoxide (AIP), and/orzirconium 1-propoxide (ZIP).

In accordance with certain embodiments, the metal oxide nanocrystalscomprise at least one of: titania, zirconia, alumina, iron oxide, zincoxide, tin oxide, beryllia, noble metal oxide, platinum group metaloxide, hafnia, molybdenum oxide, tungsten oxides, rhenium oxides,tantalum oxides, niobium oxides, vanadium oxide, chromium oxides,scandium oxides, yttria, lanthanum oxides, ceria, thorium oxides,uranium oxides, other rare earth oxides, and combinations thereof.

In accordance with certain embodiments, the suspension has a final solidcontent in a range of about 0.05% to about 10% by weight, or of up toabout 20% by weight. The suspension can be dispersed, for example,within a droplet having a diameter that is between about 0.1 μm andabout 10 mm, or between about 0.5 μm and about 5 mm, or between about 1μm and about 1 mm.

In accordance with certain embodiments, the compound product is a thinfilm deposited onto a surface, such as a surface of a substrate. In somesuch embodiments, the method also includes suspending the substrate inthe suspension prior to the solvent evaporation.

In accordance with certain embodiments, the templating particles includea colloidal suspension of spherical, elongated, concave, amorphous, orfacetted particles made from polymer, metal, metal oxide, supramolecularaggregates, crystals of organic inorganic and organometallic compounds,or salts. For example, the templating particles can include a colloidalsuspension of polymeric spheres. In some implementations, the templatingparticles can also comprise metal nanoparticles.

In accordance with certain embodiments, the photonic structure comprisestitania. The photonic structure can be formed as a film, a brick, or aspherical particle. The photonic structure can be crack-free for atleast 10,000 repeat units thereof.

In accordance with certain embodiments, the method also includescombining the metal oxide nanocrystals and the templating particles withfunctional nanoparticles, which may include one or more metals (e.g.,metal nanoparticles).

In accordance with certain embodiments, the inverse opal comprises atleast one of: a titania-gold nanoparticle inverse opal film, atitania-platinum nanoparticle inverse opal film, a titania-palladiumnanoparticle inverse opal film, titania-gold nanoparticle inverse opalphotonic bricks, and spherical titania-gold nanoparticle inverse opalparticles.

Templated structures described herein can be useful in structuralpigments, cosmetic products, pharmaceutical products, edible products,drug delivery device/mechanisms, fluidic devices, cooling devices,tissue engineering, membranes, sensors, filtration, sorption/desorption,support media, acoustic devices, batteries, fuel cells, photoactivecatalysts, catalytic mediums or supports, coherent scattering media,patterned structure fabrication, light emitters, random lasing or otheroptical applications, such as smart displays or other electrochromicmaterials.

In some implementations, the coassembly method includes combining analkyl ammonium compound with a metal alkoxide to form MO nanocrystals,e.g., titanium dioxide (TiO₂) nanocrystals, which are combined with apolymeric colloid to form a suspension. Solvent is evaporated from thesuspension to form an intermediate product, which then undergoescalcination to produce an inverse opal.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the present invention willbe apparent upon consideration of the following detailed description,taken in conjunction with the accompanying drawings, in which likereference characters refer to like parts throughout, and in which:

FIG. 1 is a diagram of a coassembly method for synthesizing photonicstructures utilizing metal oxide nanocrystals, in accordance withcertain embodiments.

FIG. 2A illustrates a titanium(IV) bis(ammonium lactato) dihydroxide(TiBALDH) sol-gel precursor and the resulting microstructure (schematicand scanning electron microscope (SEM) image) when the TiBALDH is usedto co-assemble an inverse opal.

FIG. 2B illustrates a crystalline metal oxide (MO) nanoparticle and theresulting microstructure (schematic and scanning electron microscopy(SEM) image) when the MO nanoparticle is used to co-assemble an inverseopal.

FIG. 2C illustrates a crystalline MO nanocrystal with amorphous phaseand the resulting microstructure (schematic and scanning electronmicroscope (SEM) image) when the crystalline MO nanocrystal withamorphous phase is used to synthesize an inverse opal, in accordancewith certain embodiments.

FIG. 2D is a plot showing the proportion of amorphous phase tocrystalline phase in metal oxide nanocrystals, versus the ratio(R_(TIP/TMAH)) of titanium isopropoxide (TIP) to tetramethylammoniumhydroxide (TMAH) used to prepare them, in accordance with someembodiments.

FIGS. 3A-3C are transition electron microscope (TEM) images ofas-synthesized titania nanocrystal precursors made with different ratiosof titanium precursor, in accordance with certain embodiments.

FIGS. 4A-4B are SEM images of large-area, crack-free titania inverseopal films using 420 nm PS colloids on (ITO coated PET) conductiveplastic substrate using nanocrystalline titania, in accordance withcertain embodiments.

FIGS. 5A-5B are SEM images of an alumina inverse opal film on a siliconsubstrate, fabricated using Al₂O₃ nanocrystals, in accordance withcertain embodiments.

FIGS. 5C-5D are SEM images of a zirconia inverse opal film on a siliconsubstrate, fabricated using ZrO₂ nanocrystals, in accordance withcertain embodiments.

FIG. 6A is an energy dispersive X-ray spectroscopy (EDS) spectra of analumina inverse opal film, in accordance with certain embodiments.

FIG. 6B is an EDS spectra of a zirconia inverse opal film, in accordancewith certain embodiments.

FIG. 7A is a TEM image of aluminum oxide (Al₂O₃) synthesized with analuminum isopropoxide (AIP):TMAH ratio R_(AIP/TMAH) of about 0.5, inaccordance with certain embodiments. Inset (a) of FIG. 7A shows thecrystalline structure of an individual Al₂O₃ nanocrystal.

FIG. 7B is a TEM image of zirconium oxide (ZrO₂) synthesized with azirconium 1-propoxide (ZIP):TMAH ratio of about 1.0, in accordance withcertain embodiments.

FIG. 8A is a SEM image of a TiO₂ inverse opal film, made usingnanocrystals synthesized with a TIP:TMAH ratio R_(TIP/TMAH) of about0.5, in accordance with certain embodiments.

FIG. 8B is a higher magnification SEM image of the TiO₂ inverse opalfilm of FIG. 17A, in accordance with certain embodiments.

FIG. 8C is a Raman spectrum of a TiO₂ inverse opal film aftercalcination at 500° C., the Raman spectrum indicating an anatasestructure, in accordance with certain embodiments.

FIG. 8D is an x-ray diffractometry (XRD) spectrum of an anatase TiO₂inverse opal film synthesized with a TIP:TMAH ratio R_(TIP/TMAH) ofabout 0.5, the TiO₂ inverse opal film disposed on a silicon substrateafter calcination at 500° C., in accordance with certain embodiments.

FIG. 9A is a SEM image of a TiO₂ inverse opal film synthesized with NC,in accordance with certain embodiments.

FIG. 9B is a SEM image of the TiO₂ inverse opal film of FIG. 9A, takenat a lower magnification than in FIG. 9A, in accordance with certainembodiments.

FIG. 9C is a SEM image of the TiO₂ inverse opal film of FIG. 9A, takenat a lower magnification than in FIG. 9A, in accordance with certainembodiments.

FIG. 10A shows SEM images (with higher magnification on the right) of anAl₂O₃ inverse opal, synthesized from aluminum isopropoxide—a sol-gelprecursor—in accordance with certain embodiments.

FIG. 10B shows SEM images (with higher magnification on the right) of anAl₂O₃ inverse opal, synthesized from Al₂O₃ nanocrystals, the Al₂O₃nanocrystals synthesized with an AIP:TMAH ratio R_(AIP/TMAH) of about0.5, in accordance with certain embodiments.

FIG. 11A is a SEM image of a ZrO₂ inverse opal film, synthesized fromnanocrystals synthesized with a Zr1P:TMAH ratio R_(Zr1P/TMAH) of about1, in accordance with certain embodiments.

FIG. 11B is a higher magnification SEM image of the ZrO₂ inverse opalfilm of FIG. 11A, in accordance with certain embodiments.

FIG. 12A is a SEM image of a TiO₂ inverse opal photonic brick, inaccordance with certain embodiments.

FIG. 12B is a higher magnification SEM image of the TiO₂ inverse opalphotonic brick of FIG. 12A, zoomed in on the edge of an individualphotonic crystal particle.

FIG. 13A is a SEM image of a TiO₂ inverse opal spherical microparticle,in accordance with certain embodiments.

FIG. 13B is an optical microscope image of the same microparticles shownin FIG. 13A. Each microparticle is ˜17 μm in diameter.

FIG. 13C is a SEM image of the TiO₂ inverse opal spherical microparticleof FIG. 13A, taken at a higher magnification than in FIG. 13A, inaccordance with certain embodiments.

FIG. 13D is a SEM image of the TiO₂ inverse opal spherical microparticleof FIG. 13A, taken at a higher magnification than in FIG. 13C, inaccordance with certain embodiments.

FIG. 14A is a SEM image of a TiO₂ inverse opal film incorporating goldnanoparticles (Au NP), in accordance with certain embodiments.

FIG. 14B is a higher magnification SEM image of the TiO₂ inverse opalfilm incorporating Au NP of FIG. 14A.

FIG. 15A is a SEM image of spherical TiO₂ inverse opal microparticlesformed using 420 nanometer (nm) polystyrene (PS) colloids of highlyuniform size, as characterized by a polydispersity index (PDI) of 5%, inaccordance with certain embodiments.

FIG. 15B is a SEM image of a fragment of a single photonic particle ofthe spherical TiO₂ inverse opal microparticles of FIG. 15A, inaccordance with certain embodiments.

FIG. 15C is a reflection spectrum acquired from a single photonicparticle, where inset (a) shows an optical image of the particles ofFIG. 15A, in accordance with certain embodiments.

FIG. 16A is a TEM image of a PS colloid modified with 5 nm Au NP to forma raspberry-shaped particle, in accordance with certain embodiments.

FIG. 16B is a TEM image of a PS colloid modified with 7 nm platinum (Pt)nanoparticles to form a raspberry-shaped particle, in accordance withcertain embodiments.

FIG. 16C is a TEM image of a PS colloid modified with 10 nm palladium(Pd) nanoparticles to form a raspberry-shaped particle, in accordancewith certain embodiments.

FIG. 16D is a SEM image of a TiO₂ inverse opal structure withincorporated Au NP, made from the raspberry-shaped particle of FIG. 16A,in accordance with certain embodiments.

FIG. 16E is a SEM image of a TiO₂ inverse opal structure withincorporated Pt nanoparticles, made from the raspberry-shaped particleof FIG. 16B, in accordance with certain embodiments.

FIG. 16F is a SEM image of a TiO₂ inverse opal structure withincorporated Pd nanoparticles, made from the raspberry-shaped particleof FIG. 16C, in accordance with certain embodiments.

FIG. 17 is a plot of methyl orange concentration (in ppm) versus time,showing catalytic degradation of methyl orange by a TiO₂ inverse opalupon exposure to ultraviolet (UV) light, in accordance with certainembodiments.

FIG. 18A is an optical image of a silica-based inverse opal (IO) paintwhen dry, in accordance with some embodiments.

FIG. 18B is an image of the silica-based IO paint of FIG. 18A, when wet,in accordance with some embodiments.

FIG. 18C is a plot of the % reflectance for both the dry silica-based IOpaint of FIG. 18A, and for the wet silica-based IO paint of FIG. 18B.

FIG. 18D is an optical image of a titania-based inverse opal (IO) paintwhen dry, in accordance with some embodiments.

FIG. 18E is an image of the titania-based IO paint of FIG. 18D, whenwet, in accordance with some embodiments.

FIG. 18F is a plot of the % reflectance for both the dry titania-basedIO paint of FIG. 18D, and for the wet titania-based IO paint of FIG.18E.

DETAILED DESCRIPTION OF THE INVENTION

Inverse opals are ordered, porous structures formed from colloidalcrystals, and this structuration provides them with many properties. Inparticular, their porosity facilitates wetting and fluidics studies andapplications, and their periodicity facilitates optical and photonicstudies and applications. Furthermore, altering their composition canprovide inverse opals with additional chemical functionality. Inverseopals are typically comprised of polymers, metals, or metal oxides, andthe specific material can be tailored for the application, for exampleby making colorimetric sensors using stimuli-responsive materials,making porous catalysts using catalytically active materials, or makingelectrodes using electroactive materials.

The formation of inverse opal films of titania, alumina, zirconia andother non-silica metal oxide compounds has typically been based mostlyon a three-step method. First, a sacrificial direct opal template isformed using colloidal particles, such as polymeric colloidal particles.Then, the preformed direct opal structure is infiltrated (or“backfilled”) with a metal oxide precursor to form a matrix around thedirect opal structure. Transition metal oxide inverse opals have beenmade previously with a variety of backfilling methods, includingdip-coating, dropcasting, spin coating, or vapor-phase deposition. Then,the templating colloidal particles forming the direct opal structure areremoved, leaving behind the metal oxide matrix. For example,calcination, which promotes hydrolysis, crystallization, and sinteringof the matrix, in addition to removal of the templating polymericcolloids, is commonly used. While major success has been achieved inproducing defect-free and crack-free inverse opal silica structuresusing this methodology, non-silica metal oxide inverse opals, e.g.,titania, alumina, zirconia and their mixtures show significant defects,such as cracks, and no methods have been developed so far to overcomethis problem.

Examples of metal oxide precursors used in the conventional methoddescribed above include sol-gel precursors (e.g., water-solubletitanium(IV) bis(ammonium lactato) dihydroxide (TiBALDH), as well ashighly reactive titanium alkoxides for titania; water-soluble aluminumalkoxides stabilized with acetyl acetone for alumina; and highlyreactive zirconium alkoxides that can be stabilized with acetyl acetonefor zirconia) and various oxide nanoparticles (both commerciallyavailable as well as synthesized precursors).

The traditional approach outlined above has not, therefore, beensuitable for the creation of large-scale crack-free inverse opals ofmetal oxides. Rather, conventional methods often result in crackedstructures due to natural crack formation during the drying of thecolloidal crystal film (e.g., via shrinkage), and/or due to asubstantial change in the density of the matrix during drying andcrystallization (e.g., for crystalline oxides). For example, FIG. 2Aillustrates a titanium(IV) bis(ammonium lactato) dihydroxide (TiBALDH)sol-gel precursor and the resulting microstructure (schematic andscanning electron microscope (SEM) image) when the TiBALDH is used tosynthesize an inverse opal, in accordance with certain embodiments. FIG.2B illustrates a crystalline metal oxide (MO) nanoparticle and theresulting microstructure (schematic and scanning electron microscopy(SEM) image) when the MO nanoparticle is used to synthesize an inverseopal, in accordance with certain embodiments. Cracks are clearly visiblein the microstructures of FIGS. 2A and 2B.

When backfilling direct opals, these cracks in the opal can undesirablybecome filled with matrix material. Furthermore, a single infiltrationstep can be insufficient to fill all the voids within the templatingcolloidal structure, and therefore is usually repeated via multiplefilling/infiltration steps. Multiple filling/infiltration steps canfurther lead to the formation of non-infiltrated voids within the matrixas well as to the formation of non-templated overlayers. Moreover, manysol-gel precursors have a high reactivity to moisture, and are limitedto being processed in inert conditions. In addition, since theconventional approach is based on the pre-existing templating colloidalassemblies, it lacks the flexibility to dynamically control theself-assembly process, making it impossible to achieve certainstructures or to incorporate functional components above a certain size.

The inventors previously developed a co-assembly method using a sol-gelsilica precursor (U.S. Patent Application Publication Number2011/0312080). However, such precursors for other metal oxides lack oneor more of the properties needed to produce high quality structures,such as: the ability to prevent destabilization of the colloidaldispersion (e.g., flocculation, formation of aggregates, or inducingdisorder), the ability to avoid drying- or calcination-induced cracks,for example by matching the kinetics between the colloidal assembly andhydrolysis/polymerization of the sol-gel precursor, and the ability toavoid excessive densification of the oxide phase during theamorphous-to-crystalline transition of the metal oxide. Moreover,traditional precursors can be incompatible with the incorporation ofadditional components (e.g. functional nanoparticles, nanocrystals,quantum dots, etc.), under which circumstances the surface chemistry andcharges on the precursor are extremely important.

The present disclosure describes inverse opal fabrication methods thatovercome shortcomings of the conventional approaches outlined above, forexample by facilitating the formation of highly ordered metal oxidecompound opals through a one-step co-assembly procedure using aqueoussolutions of specifically designed MO nanocrystals and templatingpolymeric colloids, leading to high-quality inverse opal structures. Forthin films of inverse opals, they can be devoid of lateral cracks above10,000 repeat units of the templating spheres, which is at least 10times larger than conventional methods. Methods described herein producehigh quality, large area inverse opal films and inverse opal powders,and can be used in combination with microfluidic techniques to formphotonic micro-particles (e.g., of a spherical shape). Methods describedherein also allow for the incorporation of dopants, such as metalnanoparticles, quantum dots, and various pigments, into the oxide matrixof the inverse opals to achieve color purification and/or saturation, aswell as to impart additional functionalities, such as catalyticactivity, load and release of guest compounds, or unique light emissioncapability. In some embodiments, the inverse opal fabrication methoddoes not include the use of a sol-gel precursor.

In the present disclosure, different co-assembly precursors areemployed, and a variety of more widely-applicable methods of makingprecursors for co-assembly of high-quality inverse opals from a varietyof transition metal oxides is set forth. For example, FIG. 2Cillustrates a MO nanocrystal with a crystalline core and amorphousphase, prepared according to methods discussed herein, and the resultingmicrostructure (schematic and scanning electron microscope (SEM) image)when the MO nanocrystal with a crystalline core and amorphous phase isused to synthesize an inverse opal. As can be seen in FIG. 2C, theresulting microstructure of the inverse opal thin film is crack-free forat least about 10,000 repeat units of the photonic crystal structure.

A number of optical, chemical, and sensing applications are facilitatedby defect-free inverted colloidal crystals, and the properties of theseinverse opal structures are further expanded by controlling theircomposition. High-quality, crack-free silica inverse opals with minimaldefects can be self-assembled using colloidal crystallization in thepresence of a sol-gel precursor, however, this co-assembly processremains challenging for transition metal oxide inverse opals. Thepresent disclosure describes methods for assembling highly ordered,crack-free inverse opals by controlling the state of the matrixprecursor, using the synthetic conditions of transition metal oxidenanocrystals to control the surface charge and crystallinity of theprecursor. Some embodiments relate to titania, however the process canbe extended to other oxides, such as metal oxides, and other materialclasses, as well as to other morphologies and templating structures.

Photonic Structures

Photonic structures that can be produced according to methods describedherein include highly ordered inverse opal structures with a variety ofmorphologies, such as thin films, bricks, balls, and bulk materials. Incertain embodiments, these inverse opal structures are crack-free for atleast 10,000 repeat units. In certain embodiments, these inverse opalstructures are crack-free for at least 1,000 repeat units. In certainembodiments, these inverse opal structures are crack-free for at least100 repeat units. In certain embodiments, these inverse opal structuresare crack-free for at least 5 repeat units.

In certain embodiments, the inverse opal structures are composed of, orsubstantially of, a metal oxide matrix and air holes.

Some other exemplary structures include “compound opals” whereincolloidal particles are present as well as the matrix component. Manydifferent types of colloidal particles can be utilized. The colloids canbe made from various materials or mixtures of materials. In certainembodiments, the materials are metals, such as gold, palladium,platinum, tin, silver, copper, rhodium, ruthenium, rhenium, titanium,osmium, iridium, iron, cobalt, nickel or combinations thereof. Incertain embodiments, the materials are semiconductor materials, such assilicon, germanium, silicon doped with group III or V elements,germanium doped with group III or V elements, tin doped with group IIIor V elements, and combinations thereof. In certain embodiments, thematerials include catalysts for chemical reactions. In certainembodiments, the materials are oxides, such as silica, titania,zirconia, alumina, iron oxide, zinc oxide, tin oxide, beryllia, noblemetal oxide, platinum group metal oxide, hafnia, molybdenum oxide,tungsten oxides, rhenium oxides, tantalum oxides, niobium oxides,vanadium oxide, chromium oxides, scandium oxides, yttria, lanthanumoxides, ceria, thorium oxides, uranium oxides, other rare earth oxides,and combinations thereof. In certain embodiments, the materials aremetal sulfides, metal chalcogenides, metal nitrides, metal pnictides,and combinations thereof. In certain embodiments, the materials areorganometallics, including various metal organic frameworks (MOFs),inorganic polymers (such as silicones), organometallic complexes, andcombinations thereof. In certain embodiments, the colloids are made fromorganic materials, including polymers, natural materials, and mixturesthereof. In certain embodiments, the material is a polymeric material,such as poly(methyl methacrylate) (PMMA), other polyacrylates, otherpolyalkylacrylates, substituted polyalkylacrylates, polystyrene (PS),poly(divinylbenzene), poly(vinylalcohol) (PVA), and hydrogels. Otherpolymers of different architectures can be utilized as well, such asrandom and block copolymers, branched, star and dendritic polymers, andsupramolecular polymers. In certain embodiments, the material is anatural material, such as a protein- or polysaccharide-based material,silk fibroin, chitin, shellac, cellulose, chitosan, alginate, gelatin,and mixtures thereof.

Formation of Inverse Opal Films

In some implementations described herein, a one-pot procedure is used tosynthesize MO nanocrystals in conjunction with colloidal particles toform a photonic structure. FIG. 1 is a diagram of a coassembly method100 for synthesizing photonic structures, in accordance with certainembodiments. As shown in FIG. 1, a metal complex 102 is combined with analkyl ammonium compound 104 to form MO nanocrystals 106. The metalcomplex can be a sol-gel precursor, for example a metal alkoxide such astitanium isopropoxide (TIP). The alkyl ammonium compound 104 can be aquaternary ammonium salt, for example tetramethyl ammonium hydroxide(TMAH). The nanocrystals 106 are then combined with colloidal particles108 (e.g., polymeric colloidal particles), also referred to herein astemplating particles (which can be spherical) to form a suspension 110that includes a solvent (e.g., water). The solvent is evaporated fromthe suspension 110, thereby forming an intermediate product 112 thatsubsequently undergoes calcination to produce an inverse opal 114.

In certain embodiments, the suspension has a final solid content of upto about 20% by weight, for example in a range of about 0.05% to about10% by weight. In certain embodiments, the suspension has a final solidcontent of up to 30 w %. In certain embodiments, the suspension has afinal solid content of up to 40 w %. In certain embodiments, thesuspension has a final solid content of up to 50 w %. In certainembodiments, the suspension has a final solid content of up to 60 w %.In certain embodiments, the suspension has a final solid content of upto 70 w %. In certain embodiments, the suspension has a final solidcontent of up to 80 w %. In certain embodiments, the suspension has afinal solid content of up to 90 w %. In certain embodiments, thesuspension has a final solid content of up to 100 w %.

In certain embodiments, sizes (e.g., diameters) of the templatingparticles can range from about 5 nm to several tens or hundreds ofmicrons. Some exemplary sizes include about 100 nm to about 1000 nm toprovide specific optical properties and/or improved assemblycharacteristics that are not largely affected by gravity. In certainembodiments, the size may range from about 100 nm to about 500 nm. Aswill be apparent to one of skill in the art, many types of sacrificialparticles can be utilized.

In certain embodiments, the colloidal particles can be decorated withnanoparticles. In certain embodiments, the nanoparticles can includemetal (e.g., gold, silver, platinum, palladium, ruthenium, rhodium,cobalt, iron, nickel, osmium, iridium, rhenium, copper, chromium,bimetals, metal alloys, and the like and combinations thereof)nanoparticles, semiconductor (e.g., silicon, germanium, and the like,pure or doped with elements or compounds of group III or V elements, andcombinations thereof) nanoparticles, metal oxide (e.g., silica, titania,zirconia, alumina, iron oxide, zinc oxide, tin oxide, beryllia, noblemetal oxide, platinum group metal oxide, hafnia, molybdenum oxide,tungsten oxides, rhenium oxides, tantalum oxides, niobium oxides,vanadium oxide, chromium oxides, scandium oxides, yttria, lanthanumoxides, ceria, thorium oxides, uranium oxides, other rare earth oxides,combinations thereof and the like) nanoparticles, metal sulfidenanoparticles, or combinations thereof.

In certain embodiments, selection of the desired nanoparticles can bebased on providing certain desired properties. For example, palladium,platinum, or other noble metal or metal oxide particles can providecatalytic properties, while silver, copper, or oxide (e.g., V₂O₅)nanoparticles can provide antibacterial properties. Other nanoparticles,such as semiconductor nanoparticles for semiconducting properties,magnetic nanoparticles for magnetic properties, and/or quantum dots foroptical properties, can be utilized as desired.

Synthesis of Metal Oxide (MO) Nanocrystals

Methods of metal oxide (MO) nanocrystal synthesis, according toembodiments described herein, comprise the combination of a metalalkoxide with an onium compound to form a reaction mixture. An oniumcompound, or onium ion, can be defined as a cation with univalent ormultivalent groups covalently bound to a central atom from thepnictogen, chalcogen, or halogen group (e.g., tetramethyl ammonium,phosphonium, etc). The onium compound can include one or more oniumcations with polyvalent substitutions (e.g., iminium, imidazolium, orpyridinium), pyrazolium, thiazolium, and/or have the form NR₄ ⁺ (e.g.,ammonium, pyrrolidinium), PR₄ ⁺ (e.g., phosphonium), and SR₃ ⁺ (e.g.,sulfonium), where R can be, for example, H, alkyl, or aryl.

In certain embodiments, nanocrystals produced according to methodsdescribed herein have crystalline cores as well as an amorphous materialdisposed therein or thereon (e.g., in the form of a coating, layer,shell, and/or the like). The ratio of amorphous material to crystallinematerial can be controlled by one or more parameters of the syntheticprocess, such as temperature, stirring speed, reflux conditions, refluxtime, pH, identity of base, concentration of ligand, aging time, etc.The biphasic nature of these resulting nanocrystals can have a favorableimpact on the inverse opal structures that are subsequently formed fromthem, perhaps because the amorphous material can accommodate stressesduring the assembly process, and thus help to minimize the formation ofcracks that are seen in traditional inverse opals produced usingtraditional synthesis techniques.

Metal Oxide (MO) Nanocrystals

In certain embodiments, the MO nanocrystals have a dimension (e.g., awidth or diameter) that is about 1 nm to about 50 nm. In certainembodiments, the MO nanocrystals have a crystalline core and anamorphous phase. In certain embodiments, the degree of crystallinity isestimated from thermal gravimetric analysis (TGA) measurements of theweight percent, for example by subtracting the weight percent at 500° C.from the weight percent at 120° C., and then dividing by the weightpercent at 500° C.

FIG. 2D is a plot showing the proportion of amorphous phase tocrystalline phase in metal oxide nanocrystals, versus the ratio ofTIP/TMAH used to prepare them, in accordance with some embodiments. Asshown in FIG. 2D, the degree of crystallinity is in the range of0.4-0.9, where 0.9 refers to the highest amount of amorphous material.

In certain embodiments, the MO nanocrystals can be titanium dioxidenanocrystals, aluminum oxide nanocrystals and/or zirconium oxidenanocrystals.

Incorporation of Functional Particles into Photonic Structures

The incorporation of a second material component into an inverse opalstructure can give rise to synergistic effects, in that it can yieldmaterials with improved or augmented functionalities and properties. Forexample, the incorporation of metal nanoparticles into inverse opalstructures results in the coupling of photonic and plasmonic properties,providing additional control over the optical properties. Furthermore,incorporation of metal nanoparticles may be advantageous for catalysis,greatly expanding the possible applications of these compositematerials. Metal nanoparticles have also been used for theantimicrobial, UV-absorbing, sensing, and electrocatalytic properties.Functional particles that can be incorporated into photonic structuresdescribed herein include metal (e.g., gold, silver, platinum, palladium,ruthenium, rhodium, cobalt, iron, nickel, osmium, iridium, rhenium,copper, chromium, bimetals, metal alloys, and the like and combinationsthereof) nanoparticles, semiconductor (e.g., silicon, germanium, and thelike, pure or doped with elements or compounds of group III or Velements, and combinations thereof) nanoparticles, metal oxide (e.g.,silica, titania, zirconia, alumina, iron oxide, zinc oxide, tin oxide,beryllia, noble metal oxide, platinum group metal oxide, hafnia,molybdenum oxide, tungsten oxides, rhenium oxides, tantalum oxides,niobium oxides, vanadium oxide, chromium oxides, scandium oxides,yttria, lanthanum oxides, ceria, thorium oxides, uranium oxides, otherrare earth oxides, and combinations thereof and the like) nanoparticles,metal sulfide nanoparticles, or combinations thereof.

In certain embodiments, selection of the desired functional particlescan be based on providing certain desired properties. For example, Pd orPt, other noble metal or metal oxide particles can provide catalyticproperties, while Ag, copper, or oxide (e.g., V₂O₅) nanoparticles canprovide antibacterial properties. Other nanoparticles, such assemiconductor nanoparticles for semiconducting properties, magneticnanoparticles for magnetic properties, and/or quantum dots for opticalproperties, can be utilized as desired.

Previous attempts to include metal nanoparticles during co-assembly oftitania using sol-gel precursors caused instability of the colloidalsolution, resulting in precipitation. By contrast, utilizing the metaloxide nanocrystals described here in conjunction with the metalnanoparticles may allow maintaining the stability of the suspension,allowing formation of inverse opal photonic crystal structure havingcrack-free structures that extend for 10,000 repeat units of the inverseopal structure.

In some embodiments, a substrate surface can be activated in order tocreate hydroxyl groups. The surface activation can be done throughexposure of the substrate to high temperature (e.g. calcination in air),and/or to etchants (e.g. piranha solution), and/or plasma.

In certain embodiments, a substrate can be made from a metal salt oroxide, such as silica, alumina, iron oxide, zinc oxide, tin oxide,alumina silicates, aluminum titanate, beryllia, noble metal oxide,platinum group metal oxide, titania, zirconia, hafnia, molybdenum oxide,tungsten oxide, rhenium oxide, tantalum oxide, niobium oxide, vanadiumoxide, chromium oxide, scandium oxide, yttria, lanthanum oxide, ceria,thorium oxide, uranium oxide, other rare earth oxides, and combinationsthereof.

In other embodiments, the substrate comprises a semiconductor, includingat least one of: silicon carbide, silicon, germanium, tin, silicon dopedwith a group III element, silicon doped with a group V element,germanium doped with a group III element, germanium doped with a group Velement, tin doped with a group III element, tin doped with a group Velement, and a transition metal oxide.

In other embodiments, the substrate comprises at least one of a metaland a metal alloy, examples of which include stainless steel, ferriticsteel (e.g., an iron-chromium alloy), austenitic steel (achromium-nickel alloy), copper, nickel, brass, gold, silver, titanium,tungsten, tin, aluminum, palladium, and platinum.

In certain embodiments, the substrate can be made from a ceramicmaterial, such as cordierite, Mullite, zeolite, and natural or syntheticclay.

In other embodiments, the substrate comprises a combination of compositemetal and metal oxide, such as cermet.

In still other embodiments, the substrate comprises an organic orinorganic material or combination thereof.

In still other embodiments, the substrate comprises a polymer, such aspolyurethane, and/or comprises at least one of:polyethyleneterephthalate, polystyrene, poly(methyl methacrylate),polyacrylate, polyalkylacrylate, substituted polyalkylacrylate,poly(divinylbenzene), polyvinylpyrrolidone, poly(vinylalcohol),polyacrylamide, poly(ethylene oxide), polyvinylchloride, polyvinylidenefluoride, polytetrafluoroethylene, other halogenated polymers,hydrogels, organogels, chitin, chitosan, random and block copolymers,branched, star and dendritic polymers, and supramolecular polymers.

In other embodiments, the substrate can be conductive metal oxide suchas indium tin oxide (ITO), fluorine doped tin oxide (FTO) or doped zincoxide.

In other embodiments, the substrate can be conductive polymer such aspoly(3,4-ethylenedioxythiophene (PEDOT), PEDOT-PSS (polystyrenesulfonate), or a carbon-based conductor (e.g. graphite).

In still other embodiments, the substrate comprises a natural material,for example including at least one of cellulose, natural rubber (e.g.,latex), wool, cotton, silk, linen, hemp, flax, and feather fiber.

Extension of the Procedure to Other Geometries

In certain embodiments, the photonic crystal structures described hereincan be made into numerous different geometries. For instance, thephotonic structure can be made into a film, monolith, powder, bricks,spherical particles, shell, coating, cylinder, rod, and other shapes.The structure can have a dimension (e.g., a length, thickness or radius)of from about 1 μm up to or above 1 cm.

EXAMPLES Example 1: Titania (TiO₂) Inverse Opal Structures

In accordance with certain embodiments, synthetic procedures weredesigned and optimized specifically to create precursors for theco-assembly of crack-free inverse opals. Previous precursors andattempted precursors either led to unstable suspensions of the assemblysolution or cracking of the resulting inverse opal structures (beforeand/or after the calcination stage). The one-pot procedure describedherein synthesizes nanocrystals that, by design, have an amorphous phaseintermixed with or adjacent to a crystalline phase, are produced using aminimal number of reagents, and have a desired size and surface charge.Precursors of the present disclosure reduce or eliminate instability ofthe suspension, and substantially reduce or prevent cracking of theresulting inverse opal structure.

To prepare TiO₂ nanocrystals, a magnetic stirrer and 90-mL of deionized(DI) water were added into a 250 mL flask, and the flask was placed inan ice-bath on top of a magnetic stir plate. A certain amount of TMAHsolution (generally 1-4 mL of 25% w/w aqueous TMAH solution) was addedand the solution was cooled down to ˜2° C. 1.1 mL (3.7 mmol) of titaniumisopropoxide (TIP) was dissolved in 15 mL of 2-propanol and addeddropwise (˜1 drop/sec) using a dropping funnel to the vigorously stirredaqueous solution of TMAH. After the addition was complete, the reactionmixture was left to stand for ˜10 minutes in an ice-bath and then for˜10 minutes at room temperature. The reaction mixture was then refluxedfor 6 hours, cooled, and used without cleaning. This procedure fornanocrystal synthesis is reminiscent of the procedure discussed byChemseddine and Moritz (Eur. J Inorg. Chem, 1999, 235-245), in whichnanoparticles were characterized for their size, shape, andcrystallinity using TEM and XRD, however their amorphous nature was notinvestigated or mentioned.

Synthesis of the TiO₂ nanocrystals was performed using various molarratios R_(TIP/TMAH) of TIP/TMAH (R_(TIP/TMAH)), including about 0.3,about 0.5, about 1.00, about 1.4, and about 1.9. FIGS. 3A-3C is a seriesof transmission electron microscopy (TEM) images of TiO₂ nanoparticlesfor TIP/TMAH ratios R_(TIP/TMAH) Of: (A) R_(TIP/TMAH)˜0.3; (B)R_(TIP/TMAH)˜1; (C) R_(TIP/TMAH)˜2. Both the concentration of TMAH andthe aging of the particles affect the NC morphology. Depending on theratio and aging, titania NC are formed with amorphous material. Loweramounts of TMAH and longer aging times can lead to increasedcrystallinity and less amorphous material.

To begin fabricating a TiO₂ inverse opal film, 100 μL of a 5.1 w %colloidal suspension, 250 μL of TiO₂ nanocrystals (e.g., as preparedaccording to procedure described above in this section), and 5 mL ofdeionized water were combined in a glass vial. The final solid contentof the colloidal particles was 0.10%.

Opals and inverse opals can be assembled or co-assembled viaevaporation-induced self-assembly (EISA). EISA is based on the so-called“coffee ring effect,” whereby a droplet of colloidal particles ornanoparticles dries with the particles deposited at the edge of thedroplet. In EISA, evaporation currents drive colloidal particles to theair-water interface. Above a certain critical colloid concentration, themeniscus shape causes a thin film to grow on a submerged substrate. EISAhas been used extensively to grow direct opals. By adding NC's to theassembly solution, titania-polymer compound opals can form. Depending onthe ratio (R_(TIP/TMAH)) and aging time, crack-free titania inverseopals can form.

Example 1(A): Fabrication of TiO₂ Crack-Free Inverse Opal Structures onSilicon Substrates

Silicon (Si) wafer substrates (cut to approximately 1 cm×5 cm), werecleaned in piranha solution and then vertically suspended in the glassvial containing the colloid/TiO₂ nanoparticle suspension. The water wasevaporated over a period of 1-2 days in a 65° C. oven on a pneumatic,vibration-free table to allow the deposition of a compound opal thinfilm onto the suspended substrate. The compound opal thin films werethen calcined at 500° C. for 2 hours, with a 5 hour ramp time, to removethe polymer template and to sinter the TiO₂ nanoparticle, therebyforming an inverse opal film on the Si substrate.

FIG. 8A is a SEM image of a TiO₂ inverse opal film, made using a TiO₂nanoparticle synthesized with a TIP:TMAH ratio R_(TIP/TMAH) of about0.5, in accordance with certain embodiments. FIG. 8B is a highermagnification SEM image of the TiO₂ inverse opal film of FIG. 8A.

FIG. 8C is a Raman spectrum of a TiO₂ inverse opal film, produced viacalcination at 500° C., the Raman spectrum indicating an anatasestructure. FIG. 8D is an X-ray diffraction (XRD) spectrum of a TiO₂inverse opal film synthesized with a TIP:TMAH ratio R_(TIP/TMAH) ofabout 0.5, the TiO₂ inverse opal film disposed on a silicon substrateafter calcination at 500° C., as described above. The XRD spectrumindicates that the inverse opal film has a crystalline structure that ischaracteristic of anatase TiO₂.

As can be seen in FIGS. 8A-8B, cracks are not visible in the inverseopal film fabricated according to methods of the present disclosure—noteven at low magnification. For purposes of comparison, TiO₂ inverseopals made using nanoparticles that were fabricated using the publishedsynthesis procedure of Chemseddine and Moritz are shown in FIGS. 9A-9C(at varying degrees of magnification). The TiO₂ inverse opals of FIGS.9A-9C have minor cracks, and their microstructure ordering is notoptimized. Without wishing to be bound by theory, the inventorsattribute this fact to the presence of additional charges and moleculesin the as-published procedure due to the high pH, which can disrupt thecolloidal assembly. By adjusting the synthesis procedure, the inventorshave successfully produced high-quality, crack-free inverse opalassemblies for all investigated molar ratios R_(TIP/TMAH) of TIP/TMAHabove about 0.3. According to embodiments described herein, aftercalcination, the TiO₂ of the TiO₂ inverse opals is present in theanatase phase, according to both Raman spectroscopy (FIG. 8C) and X-raydiffraction (FIG. 8D). In addition to cracks, the size and crystallinityof the nanoparticles will affect some inverse opal properties; forexample, smaller particles will lead to more grain boundaries, which canincrease catalytic activity.

Example 1(B): Fabrication of TiO₂ Inverse Opal Film on FlexibleSubstrates

Titania has several attractive inherent properties, such as its high RI,(photo)catalytic and electrochromic activities. Titania inverse opalsfilms have promising applications in the field of electrochromic (EC)devices. See, e.g., Hua Li, Guillaume Vienneau, Martin Jones, BalajiSubramanian, Jacques Robichaud and Yahia Djaoued “Crack-free 2D-inverseopal anatase TiO2 films on rigid and flexible transparent conductingsubstrates: low temperature large area fabrication and electrochromicproperties” J Mater. Chem. C, 2014, 2, 7804. Electrochromism can bedefined as the ability of a material to undergo color change induced byan external electric field. Current applications of electrochromisminclude self-darkening rear view mirrors and electrochromic windows.Ordered titania inverse opal films exhibit improved EC performance dueto significantly fast switching times and improved coloration contrasts.Titania inverse opals thus combine optical and catalytic properties ofthe material and structure, in part because the “slow photon” effectenhances photocatalytic activity due to the photonic nature of thestructure. Indeed, titania inverse opals have garnered many reports oftheir photocatalytic activity, but higher quality structures are stillnecessary to improve the slow photon effect, as well as to enable betterfundamental studies into this slow photon enhancement of photocatalysis.For electrochromic devices, an inverse opal architecture of V₂O₅, TiO₂and WO₃ has recently been reported for its significantly fast switchingtimes and improved coloration contrast. See, e.g., Zhang, J at al.“Energy Dispersive X-ray Spectroscopy Enhanced electrochromicperformance of highly ordered, macroporous WO3 arrays electrodepositedusing polystyrene colloidal crystals as template” Electrochimica Acta,2013, 99, 1; Li, L. et al. “Improved electrochromic performance ininverse opal vanadium oxide films” J Mater. Chem., 2010, 20, 7131.

Atomic layer deposition (ALD) is conventionally used for the fabricationof large area, crack-free titania inverse opals films on transparentsubstrates such as indium tin oxide (ITO) coated glass. The posttreatment of opals structures at high temperatures that is typicallyrequired in order to crystalize the matrix material and to remove thetemplating colloids limits the choice of substrates to thermally stableones. This represents a significant drawback, as the development ofwireless technologies and modern electronics requires the design ofinexpensive, lightweight, and efficient optoelectronic devices such asportable solar cells or EC devices on flexible substrates. Methodologiesdescribed herein facilitate fabrication of large-area defect-free andcrystalline titania films on transparent flexible conductive substratesusing cost efficient and straightforward fabrication method.

Large-area crack-free titania (anatase) IO films were fabricated on ITOcoated flexible polyethyleneterephthalate (ITO/PET) substrates using theapproach described in the current disclosure. The corresponding SEMimages are shown in FIGS. 4A-4B. The compound opal films were fabricatedthrough a co-assembly of titania nanocrystals (R_(TIP/TMAH)=1.35) and PScolloids (420 nm) on a pre-treated ITO/PET substrate. As apre-treatment, the ITO coated plastic substrates were cleaned using anultrasonic bath for 3 min, successively in acetone, ethanol anddeionized water, dried and plasma treated for 1 min. Following formationof the compound opal film, the PS colloids were removed by dissolutionin toluene.

Example 2: Alumina (Al₂O₃) Inverse Opal Structures

Synthesis of Al₂O₃ nanoparticles was performed as follows: Aluminumisopropoxide (AIP), a sol-gel precursor, was dissolved in 15 mL of2-propanol and added dropwise to an aqueous solution of TMAH. Thereaction mixture was then refluxed for 48 hours. FIG. 7B is a TEM imageof Al₂O₃ nanocrystal synthesized with a TIP:TMAH ratio R_(AIP/TMAH) ofabout 0.5. The inset (a) of FIG. 7B shows the crystalline structure ofan individual Al₂O₃ nanoparticle.

Synthesis of Al₂O₃ inverse opal films was performed using a colloidaldispersion containing 100 μL of a 5.1 w % colloidal suspension, 100 μLof Al₂O₃ nanocrystal solution and 5 mL of deionized water. An inverseopal prepared using a standard alumina precursor (aluminumisopropoxide—a sol-gel precursor) is shown in FIG. 10A for reference,and SEM images of Al₂O₃ inverse opals assembled from the metal oxidenanocrystal precursors shown in FIG. 7B (with a R_(AIP/TMAH) of about0.5) are shown in FIG. 10B.

Example 3: Zirconia (ZrO₂) Inverse Opal Structures

Synthesis of ZrO₂ nanoparticle was performed as follows: zirconium1-propoxide (ZIP) solution, a sol-gel precursor, was dissolved in 15 mLanhydrous 2-propanol in a glovebox. The solution was subsequentlyremoved and added dropwise to an aqueous solution of TMAH, followed by a12 hour reflux. FIG. 7C is a TEM image of ZrO₂ nanoparticle synthesizedwith a ZIP:TMAH ratio R_(ZIP/TMAH) of about 1.0.

Synthesis of ZrO₂ inverse opal films was performed using a colloidaldispersion containing 100 μL of a 5.1 w % colloidal suspension, 400 μLof ZrO₂ nanoparticle and 4.5 mL of deionized water. FIGS. 11A-11B areSEM images (at varying magnifications) of a ZrO₂ inverse opal film,synthesized as outlined above, where the ZrO₂ nanoparticle weresynthesized with a ZIP:TMAH ratio R_(ZIP/TMAH) of about 1.0 (i.e., thenanoparticle precursor shown in FIG. 7C).

Example 4: Incorporation of Gold Nanoparticles into Titania InverseOpals Through 3-Phase Co-Assembly

Gold nanoparticles (Au NP) were incorporated into inverse opal films(shown in FIG. 14A and further magnified in FIG. 14B), bricks, and balls(shown in FIG. 15A and further magnified in FIG. 15B) by adding apolyethylene glycol (PEG)-modified Au NP solution (to a final weightpercent of 0.1 to 5 w %) into a co-assembly precursor solutioncomprising an aqueous solution of polymer (420 nm polystyrene (PS))colloids with a polydispersity index (PDI) of <5% and TiO₂ NCs. Waterwas added to achieve the desired overall precursor concentrationsdescribed above for films, bricks, and photonic balls. Incorporation ofAu NP intensifies the red appearance (shown as the lighter centralregions of the nanoparticles shown in the grayscale image of FIG. 15C)of the photonic structure through selective absorption of undesiredreflections. Exemplary effects on the optical spectra are shown forphotonic balls in the reflection spectrum, acquired from a singlephotonic particle, of FIG. 15C. The photonic peak at ˜680 nm is enhancedby the Au NP absorption (˜550 nm). Inset (a) shows an optical image ofthe particles of FIG. 15A.

Example 5: Incorporation of Gold, Platinum, and Palladium Nanoparticlesinto Titania Inverse Opals Through 2-Phase Co-Assembly Using RaspberryParticles

The inventors have previously reported on decorated particles (WO2014/210608). Decorated particles, or “raspberry-shaped particles,” arecomposite metal-polymer colloidal particles comprising metalnanoparticles that are covalently bound to a chemically modified surfaceof polystyrene (PS) colloids, with homogeneous metal nanocrystalsdistribution at the pore surface. FIGS. 16A-16C are TEM images ofraspberry particles formed with a PS colloid modified with: 5 nm Aunanocrystals, 7 nm platinum (Pt) nanoparticles, and 10 nm palladium (Pd)nanoparticles, respectively. These composite PS—metal nanoparticlescolloids can be employed as template particles in the assembly ofinverse opal scaffolds. Inverse opals can be assembled using a protocolsimilar to the co-assembly of TiO₂ inverse opals described herein, butwith raspberry-shaped particles substituted for the unfunctionalized PScolloids. A colloidal suspension of raspberry-shaped particles was mixedwith 250 μL of TiO₂ nanocrystal solution and deionized water to achievea 0.1% final solid content of the colloidal suspension. Si wafersubstrates (cut to approximately 1 cm×5 cm), cleaned in piranhasolution, were vertically suspended in a vial containing the raspberryparticles/TiO₂ nanoparticle suspension. The water was evaporated over aperiod of 1-2 days in a 65° C. oven on a pneumatic vibration-free table,to allow the deposition of a thin film onto the suspended substrate. Thecompound opal films were then calcined at 500° C. for 2 hours, with a 5hour ramp time, to remove the polymer template and to sinter the TiO₂NC. SEM images of exemplary inverse opal structures with incorporatedmetal nanoparticles are shown in FIGS. 16D-16F (where the metalnanoparticles is Au, Pt and Pd, respectively).

In some embodiments, the metal nanoparticle incorporation methoddescribed above yields composite inverse opals with metal nanoparticlesfound exclusively at the air/metal oxide (e.g., TiO₂) interface of theinverse opal matrix. In such configurations, each metal nanoparticle isaccessible/exposed to chemical reagents or analytes that are introducedinto the scaffold via liquid or gas phase, which is advantageous forapplications such as catalysis and surface-enhanced Raman scattering(SERS). In addition, the confinement of plasmonic nanoparticles (e.g.,Au NP) to the surface of the inverse opal scaffold induces a controlledspectral modification of the inverse opal reflection, making thismaterial a versatile platform for photonic/plasmonic colorimetricsensing and effect pigmentation with angle-independent colorationresulting from the metal nanoparticles absorption and iridescenceimparted by the inverse opal's nanoscale periodicity.

Example 6: Photonic Crystal Bricks

Assembly of TiO₂ inverse opal photonic bricks, or “freeform” photonicbricks were grown on the inner walls of 20 mL glass vials containingco-assembly solution. The thickness of the photonic bricks was tunableby adjusting the concentration of the precursor solution. For example,in one implementation, 300 μL of a 5.1 w % colloidal suspension, 750 μLof TiO₂ nanoparticles and 5 mL of deionized water were combined in aglass vial. The final solid content of the colloidal suspension was0.3%. The solvent content of the colloidal suspension was evaporatedover a period of 1-2 days in a 65° C. oven on a pneumatic vibration-freetable, to allow the deposition of a film onto the inner walls of thevial. In this colloidal concentration regime, the formation of photonicbricks occurs spontaneously due to natural cracking of the filmdeposited on the wall, thereby facilitating particulate release forcollection during calcination (e.g., at 500° C.) and sintering. An SEMimage of “freeform” TiO₂ opal photonic bricks is shown in FIG. 12A, anda higher magnification SEM image of the TiO₂ inverse opal photonic brickof FIG. 12A, zoomed in on the edge of an individual photonic crystalparticle.

Example 7: Photonic Crystal Balls

The formation of spherical inverse opal particles, or “photonic balls,”in accordance with the present disclosure was performed as follows: Anaqueous solution containing ˜0.5 wt-% of polystyrene colloids (256 nm,PDI˜5%) and ˜0.7 wt-% of TiO₂ nanoparticles was emulsified using at-junction microfluidic device with 150 μm channel width. The continuousphase contained 0.5 wt-% of a surfactant in Novec 7500 (3M). Uponevaporation of water from the droplets, the resulting sphericalmicroparticles were calcined at 600° C. for two hours. SEM images of theresulting TiO₂ inverse opal spherical microparticles are shown in FIGS.13A and 13C-13D (at varying magnification), and an optical microscopeimage of the TiO₂ inverse opal spherical microparticles is shown in FIG.13B. The TiO₂ inverse opal spherical microparticles have an iridescentappearance that is dominated by a blue color, when white light isreflected in a specular configuration. Further details concerningphotonic balls can be found in U.S. Patent Application PublicationNumber 2014/0254017.

In the foregoing examples, the final solid content of the suspensionprior to calcining is typically about 1-10 w % initially for photonicballs, about 0.1 w % for films, and about 0.3 w % for shards. In eachcase, the water or other solvent component of the suspension evaporatesover time, and therefore the solid content can increase over time.

Applications

Inverse opals made according to certain embodiments herein can enableand/or enhance a variety of applications. For example, in catalysis,TiO₂ is known to oxidize organic molecules upon irradiation with UVlight. These TiO₂ inverse opals were tested for their photocatalyticactivity using methyl orange as a test molecule. The concentration ofmethyl orange in solution was monitored with UV-vis spectroscopy. FIG.17 is a plot of methyl orange concentration (in ppm) versus time,showing catalytic degradation of methyl orange by a TiO₂ inverse opalupon exposure to ultraviolet (UV) light.

TiO₂, Al₂O₃, ZrO₂, and other metal oxide inverse opal films, powders andmicroparticles described herein combine the benefits of the inverse opalstructure (including both periodicity and porosity) with the chemicaland physical properties of the oxides and their mixtures (includingrefractive index, absorption properties, catalytic activity,electroactivity, or strength). Metal oxide inverse opals of the presentdisclosure can be useful for applications such as:

-   -   structural color dyes in paint formulations (with the ability to        retain color in solutions)    -   structural color dyes in cosmetic products, with additional        functionality such as antimicrobial or sunscreen properties    -   heterogeneous/dispersive catalytic substrates or supports    -   solar cells and fuel cells (with the metal oxide as the active        or support medium)    -   other electrode uses, such as photoelectrochemical cells,        capacitors, and batteries    -   sensing substrates, for example in colorimetric indicators or        for Raman enhancement    -   drug loading and release vehicles    -   filters and membranes, for separation or air filtration    -   photonic applications, such as lasing and waveguiding    -   other optical applications, such as smart displays or other        electrochromic materials    -   substrates for microelectronics applications    -   coatings for cooling devices

For example, the catalytic activity of titania inverse opals isdemonstrated in FIG. 17. Titania inverse opal particles in accordancewith some embodiments is shown to catalyze the degradation of methylorange under UV light.

As another example, the color retention performance of titania inverseopal-based paint is compared with that of silica inverse opal paint inFIGS. 18A-18F. FIG. 18A is an optical image of a silica-based inverseopal (IO) paint when dry, in accordance with some embodiments. FIG. 18Bis an image of the silica-based IO paint of FIG. 18A, when wet, inaccordance with some embodiments. FIG. 18C is a plot of the %reflectance for both the dry silica-based IO paint of FIG. 18A, and forthe wet silica-based IO paint of FIG. 18B.

FIG. 18D is an optical image of a titania-based inverse opal (IO) paintwhen dry, in accordance with some embodiments. FIG. 18E is an image ofthe titania-based IO paint of FIG. 18D, when wet, in accordance withsome embodiments. FIG. 18F is a plot of the % reflectance for both thedry titania-based IO paint of FIG. 18D, and for the wet titania-based IOpaint of FIG. 18E.

Embodiments described herein, comprising the formation of high-qualitystructures through coassembly within droplets, can be used for directprinting of TiO₂ photonic structures using inkjet printing technology.

As used herein, the terms “about” and “approximately” generally meanplus or minus 10% of the value stated, e.g., a value of about 250 wouldinclude 225 to 275, and about 1,000 would include 900 to 1,100.

Upon review of the description and embodiments provided herein, thoseskilled in the art will understand that modifications and equivalentsubstitutions may be performed in carrying out the invention withoutdeparting from the essence of the invention. Thus, the invention is notmeant to be limiting by the embodiments described explicitly above.

1. (canceled)
 2. An inverse photonic structure comprising a firstcomponent; and a metal oxide matrix, wherein the metal oxidenanocrystals comprise at least one of: a transition metal, titania,zirconia, alumina, iron oxide, zinc oxide, tin oxide, beryllia, a noblemetal oxide, platinum group metal oxides, hafnia, molybdenum oxide,tungsten oxide, rhenium oxide, tantalum oxide, niobium oxide, vanadiumoxide, chromium oxide, scandium oxide, yttria, lanthanum oxide, ceria, arare earth oxide, thorium oxide, uranium oxide, other rare earth oxides,or a combination thereof wherein the inverse photonics structure iscrack-free for at least 10,000 repeat units.
 3. The inverse photonicstructure of claim 2, wherein the metal oxide matrix comprises atransition metal.
 4. The inverse photonic structure of claim 2, whereinthe metal oxide matrix is selected from a group consisting of titania,alumina, and zirconia.
 5. The inverse photonic structure of claim 2,wherein the metal oxide matrix comprises titania.
 6. The inversephotonic structure of claim 2, wherein the first component is an airhole.
 7. The inverse photonic structure of claim 2, wherein the airholes have diameters of 100 nm to 1000 nm.
 8. The inverse photonicstructure of claim 6, wherein the inverse photonic structure includesnanoparticles at the interface of the metal oxide matrix and the airholes.
 9. The inverse photonic structure of claim 6, wherein thenanoparticles are selected from a group consisting of metalnanoparticles, semiconductor nanoparticles, metal oxide nanoparticles,metal sulfide nanoparticles, and combinations thereof.
 10. The inversephotonic structure of claim 2, wherein the first component is acolloidal particle.
 11. The inverse photonic structure of claim 10,wherein the colloidal particle comprises a material selected from thegroup consisting of metals, semiconductor materials, oxide, sulfide,organometallic complexes, organic materials, polymers, naturalmaterials, and combinations thereof.
 12. The inverse photonic structureof claim 10, wherein the colloidal particles have diameters of 100 nm to1000 nm.
 13. The inverse photonic structure of claim 10, wherein thecolloidal particles are decorated with nanoparticles.
 14. The inversephotonic structure of claim 13, wherein the nanoparticles are selectedfrom a group consisting of metal nanoparticles, semiconductornanoparticles, metal oxide nanoparticles, metal sulfide nanoparticles,and combinations thereof.
 15. The inverse photonic structure of claim 2,wherein the metal oxide incorporates functional particles.
 16. Theinverse photonics structure of claim 15, wherein the functionalparticles are selected from a group consisting of metal nanoparticles,semiconductor nanoparticles, metal oxide nanoparticles, metal sulfidenanoparticles, and combinations thereof.
 17. The inverse photonicstructure of claim 2, wherein the inverse photonic structure is aninverse opal structure.
 18. The inverse photonic structure of claim 2,wherein the inverse photonic structure is a thin film deposited onto asurface.
 19. The inverse photonic structure of claim 2, wherein theinverse photonic structure is formed as one of a film, a brick, aspherical particle, or a particle of a complex shape.